1995A&A...302..497G

We analyse HST GHRS observations of the interstellar medium in the direction to ε CMa, the strongest EUV source in the sky, located in a region deficient in neutral gas. The line of sight contains three main absorption components resolved in the high resolution Ech B observations. Three weak components are also detected. We derive their heliocentric velocities, their column densities in FeII, MgII, MgI, SIIII and CIV, upper limits for NI and HI, as well as the temperature and turbulence velocity of the three main components. The analysis leads to the following conclusions: 1) The neutral gas column density is very low indeed: less than 5x10¹⁷cm^–2. This value is considerably lower than what had been measured before and it puts constraints on the EUV ionizing flux from ε CMa. 2) Two of the main clouds are identified as the two components observed in the nearby Sirius line of sight, which shows that they lie less than 3pc away from the Sun. Beyond this distance, the remaining 180pc line of sight is thus an interstellar region with the lowest neutral mean density yet measured, with less than 2x10^–4cm^–3. 3) One of the main components is the local interstellar cloud (LIC) in which the Sun is embedded. Its column density and its temperature (∼7000K) are in agreement with the lines of sight to Sirius and Capella. We derive a high electron density n_e=0.09^+.23_–.07cm^–3 implying a high ionization fraction which could reach 50%. 4) High ionization state elements like SiIII and CIV are detected for the LIC and the third main component, showing that a hot phase is associated with these clouds. An extra absorption well described by a hot HI counterpart for the LIC is also required by the Lyman α profile fitting. We suggest that we are observing the conduction fronts between the clouds and the hot bubble in which they are imbedded.